Effect of plastic fines on the small strain stiffness of sand

ABSTRACT: The paper investigates the small strain shear stiffness (Gmax) of sand-bentonite specimens (0, 3 and 5 % bentonite by dry mass of the sand) prepared at the same skeleton void ratio (corresponding to Drsk=35-40%) using a dry pluviation technique. Based on resonant column tests performed at very small strains at increasing stress levels (25-300 kPa), it is found that the same type of relationship links the initial stiffness (Gmax) to stress level and skeleton void ratio, with the presence of fines leading to only a marginal decrease in Gmax. This effect can be ascribed to the presence of bentonite at the sand grain contacts. The paper also presents Gmax data obtained over time (up to approximately 7 days), which show that the presence of 3 % and 5 % bentonite increases the aging coefficient (NG) by a factor of 2 and 3, respectively. Finally the paper examines the values of the shear wave velocity (Vs) measured on the sand-bentonite specimens and draws attention to the potential problems in employing this soil property as an index of liquefaction resistance in soils with small percentages of highly plastic fines